1. Field of the Invention
The present invention relates to a metal-air cell, and more particularly to a metal-air cell utilizing a piezo-electric pump.
2. Description of Related Art
Secondary cells, such as a nickel-cadmium cell, a nickel-hydrogen cell or a lead storage battery are well-known. These secondary batteries are often used in small-sized electronic devices. However, with the popularization of small-sized electronic devices and the changing environment and uses surrounding such devices, there is an increasing demand for a secondary cell having a higher energy density. The lithium secondary cell has drawn attention for it's high energy density. However, due to safety problems stemming from it's high flammability, the lithium secondary cell has not become popular.
The recently-developed metal-air cell is expected to replace the nickel-cadmium cell as the next-generation secondary cell. For example, the zinc-air cell represents such a metal-air cell which is a suitable substrate for the lithium secondary cell. In the zinc-air cell, supplying a required amount of atmospheric molecular oxygen, by supplying a corresponding amount of air, to the positive electrode enables a charge/discharge reaction at the positive electrode: EQU 1/2O.sub.2 +H.sub.2 O+2e.revreaction.2OH .
Meanwhile, a charge/discharge reaction occurs at the negative electrode: EQU Zn+2OH.revreaction.ZnO+H.sub.2 O+2e .
Hence, the zinc-air cell has an overall reaction: EQU Zn+1/2O.sub.2 .revreaction.ZnO
As described above, since the zinc-air cell consumes a required amount of atmospheric molecular oxygen at the positive electrode, the conventional zinc-air cell has, in general, an electric fan equipped in the casing.
With this construction, it is difficult to attain a high net energy density because of the difficulty in reducing the volume which is consumed by the electric fan. The electric fan of the prior art zinc-air cell is similar to those used in conventional personal computers. The electric fan is approximately 50 mm by 150 mm by 200 mm and one electric fan can be connected to up to, and usually is connected to, six of the zinc-air cells. In addition, as shown in FIG. 5, the air inflow speed for the fan is highly dependent on the pressure differential between the intake and outlet valves (i.e., a small change in the pressure differential results in a large change in the air flow rate), making it difficult to control the air intake speed. Accordingly, not only is the air flow rate of the electric fan difficult to control, but all of the zinc-air cells connected to the electric fan are turned on and off and controlled as a group, even though the operator may desire to turn on or off or control only one of the zinc-air cells. Consequently, the inaccurate control of the air supply to the positive electrode of the zinc-air cell degrades the performance of the zinc-air cell. The present invention solves the above problem.